A conventional mask programmable read only memory comprises a plurality of memory cell matrices each having a plurality of memory cell blocks. Each of the memory cell blocks comprises a plurality of memory cells serially connected, and the serially connected memory cells are connected through a block selection transistor to a digit line on one side thereof, and are connected to ground on the other side thereof.
In this mask programmable read only memory, the memory cell unit is composed, for instance, in such a manner that N channel MOS enhancement transistor having a threshold voltage of 1.0 V and N channel MOS depletion transistor having a threshold voltage of -5.0 V are assigned to store binary signals in accordance with the difference of the threshold voltages which is resulted from the injection of impurity, for instance, P into the transistors.
In operation, one of the memory cell matrices is selected by a Y-decoder circuit, while one of the memory cell blocks is selected in the selected memory cell matrix by a block decoder circuit. In the selected memory cell block, one of the memory cells is accessed in accordance with the application of low level signal (0 V) by an X-decoder circuit, while non-accessed memory cells are applied with high level signal (5 V). As a result, if the accessed memory cell is the enhancement transistor, it is turned on even under the application of low level signal. On the other hand, the non-accessed memory cells are turned on regardless of the type of transistors. Thus, a potential of the digit line which is thereby connected through the turned-on memory cells including the accessed memory cell to ground is lowered, for instance, from 1.2 V to 1.1 V. This potential change is amplified in a sense amp circuit to provide information read from the accessed memory cell.
On the contrary, if the accessed memory cell is the depletion transistor, it is not turned on under the application of low level signal. Therefore, the potential change does not occur. In this manner, information stored in the memory cell unit is read upon the access of a predetermined one of the memory cells by applying a low level signal thereto, while a high level signal is applied to the non-accessed memory cells of the selected block, and to the remaining memory cells of the selected memory cell matrix and the non-selected memory cell matrices.
In this conventional mask programmable read only memory, however, there is a disadvantage in that electric field stress applied to the memory cells is large to shorten the life thereof, thereby lowering the reliability thereof, because a high level signal is applied to all word lines except for one word line which is selected at the access time by the X-decoder circuit. The same thing can be said at the stand-by time, because all the word lines are applied with a high level signal.